This invention relates to subsonic venturi proportional and isokinetic sampling methods and an apparatus, and more particularly to an arrangement for determining the gaseous constituents of exhaust utilizing a pair of subsonic venturi restrictions, one for measuring the exhaust or exhaust/air flow and the other for measuring the flow of an extracted sample, wherein the system is designed to provide both proportional and proportional isokinetic sampling.
Under present day federal regulations, the exhaust emissions from motor vehicles must not exceed specified values of certain constituent contaminants, as set forth in the Code of Federal Regulations. See, for example, Title 40 of the Code of Federal Regulations, parts 81-99, subparts A, B, D, E, F, G, K and N. See also Kaufman, U.S. Pat. No. 3,699,814. The presence of such standards has made it imperative that the exhaust emissions from vehicle engines be tested and analyzed to determine the relative amount of certain constituents therein. Much effort has gone into the development of equipment for use in this field of exhaust gas sampling, and it is now known to deliver exhaust from an internal combustion engine at an accurately controlled flow rate through a test apparatus for purposes of determining and analyzing the relative amounts of constituents therein. The general scheme of such testing is to add dilution air to the exhaust. The total volume of the mixture of exhaust and dilution air must be measured. A continuously proportional sample of volume must be collected and is stored for subsequent analysis of constituents such as hydrocarbons, carbon monoxide and NO.sub.x. Mass emissions are determined from the sample concentrations and total flow over the test period.
Once such system for analyzing samples from exhaust gases is set forth in U.S. Pat. No. 3,699,814 to Kaufman entitled, "Gas Sampler," issued Oct. 24, 1972. The disclosure of this patent is incorporated herein by reference. The Kaufman patent is directed to a gaseous exhaust emission sampler which replaced the constant displacement pump of prior systems with a critical flow venturi and centrifugal blower for metering the diluted exhaust emissions at a constant volume flow.
Another system utilizes a pair of critical flow venturis for proportional sampling. An example of such a system is set forth in U.S. Pat. No. 3,817,100. In another such system, a downstream pump produces a sufficient vacuum on the bulkstream critical flow venturi exit so that the bulkstream mixture is flowing at sonic velocity, a condition which limits the bulkstream mixture to a constant mass flow rate at a given set of upstream temperature and pressure conditions measured at the bulkstream critical flow venturi inlet. A sample is extracted from the dilute bulkstream flow through another critical flow venturi in close proximity to the bulkstream critical flow venturi so that the venturis are operating under the same inlet pressure and temperature conditions. This sample critical flow venturi operates in connection with a downstream pump in the sampling line to create sonic flow, and thereby a constant mass flow rate at the measured upstream temperature and pressure conditions. Thus, the sample critical flow venturi extracts a sample for analysis at a flow rate proportional to the bulkstream flow rate.
Although this type of proportional sampling system using two critical flow venturis has certain advantages, it has a disadvantage in that it does not provide for active control of the bulkstream or sample flow rates to allow static and dynamic sampling of the bulkstream mixture. Consequently, such prior art systems cannot readily accommodate the testing of different size internal combustion engines which generate substantially different exhaust flow volumes without using different bulkstream critical flow venturis.
It is, therefore, a principal object of this invention to provide improved methods and an apparatus for sampling the emission content of exhaust from an exhaust source which provides for active flow control and proportional sampling, and may be sized for isokinetic flow conditions.
It is another object of this invention to provide methods and an apparatus for sampling the emission content of exhaust from an exhaust source which utilizes a pair of subsonic venturi restrictions that have substantially the same throat pressure for a preselected ratio of the flow rate of the exhaust to the flow rate of the extracted sample through their respective venturi throats, providing proportional sampling.